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Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" From: @(#)listen.2 8.2 (Berkeley) 12/11/93 33.\" $FreeBSD: src/lib/libc/sys/listen.2,v 1.12.2.9 2002/05/09 02:24:40 silby Exp $ 34.\" $DragonFly: src/lib/libc/sys/listen.2,v 1.2 2003/06/17 04:26:47 dillon Exp $ 35.\" 36.Dd November 3, 1995 37.Dt LISTEN 2 38.Os 39.Sh NAME 40.Nm listen 41.Nd listen for connections on a socket 42.Sh LIBRARY 43.Lb libc 44.Sh SYNOPSIS 45.In sys/types.h 46.In sys/socket.h 47.Ft int 48.Fn listen "int s" "int backlog" 49.Sh DESCRIPTION 50To accept connections, a socket 51is first created with 52.Xr socket 2 , 53a willingness to accept incoming connections and 54a queue limit for incoming connections are specified with 55.Fn listen , 56and then the connections are 57accepted with 58.Xr accept 2 . 59The 60.Fn listen 61call applies only to sockets of type 62.Dv SOCK_STREAM 63or 64.Dv SOCK_SEQPACKET . 65.Pp 66The 67.Fa backlog 68parameter defines the maximum length the queue of 69pending connections may grow to. 70If a connection 71request arrives with the queue full the client may 72receive an error with an indication of 73.Er ECONNREFUSED , 74or, in the case of TCP, the connection will be 75silently dropped. 76.Pp 77Note that before FreeBSD 4.5 and the introduction of the syncache, 78the 79.Fa backlog 80parameter also determined the length of the incomplete 81connection queue, which held TCP sockets in the process 82of completing TCP's 3-way handshake. These incomplete connections 83are now held entirely in the syncache, which is unaffected by 84queue lengths. Inflated 85.Fa backlog 86values to help handle denial 87of service attacks are no longer necessary. 88.Pp 89The 90.Xr sysctl 3 91MIB variable 92.Dq Va kern.ipc.somaxconn 93specifies a hard limit on 94.Fa backlog ; 95if a value greater than 96.Va kern.ipc.somaxconn 97or less than zero is specified, 98.Fa backlog 99is silently forced to 100.Va kern.ipc.somaxconn . 101.Sh INTERACTION WITH ACCEPT FILTERS 102When accept filtering is used on a socket, a second queue will 103be used to hold sockets that have connected, but have not yet 104met their accept filtering criteria. Once the criteria has been 105met, these sockets will be moved over into the completed connection 106queue to be accept()ed. If this secondary queue is full and a 107new connection comes in, the oldest socket which has not yet met 108its accept filter criteria will be terminated. 109.Pp 110This secondary queue, like the primary listen queue, is sized 111according to the 112.Fa backlog 113parameter. 114.Sh RETURN VALUES 115.Rv -std listen 116.Sh ERRORS 117.Fn Listen 118will fail if: 119.Bl -tag -width Er 120.It Bq Er EBADF 121The argument 122.Fa s 123is not a valid descriptor. 124.It Bq Er ENOTSOCK 125The argument 126.Fa s 127is not a socket. 128.It Bq Er EOPNOTSUPP 129The socket is not of a type that supports the operation 130.Fn listen . 131.El 132.Sh SEE ALSO 133.Xr accept 2 , 134.Xr accept_filter 9 , 135.Xr connect 2 , 136.Xr socket 2 , 137.Xr sysctl 3 , 138.Xr sysctl 8 139.Sh HISTORY 140The 141.Fn listen 142function call appeared in 143.Bx 4.2 . 144The ability to configure the maximum 145.Fa backlog 146at run-time, and to use a negative 147.Fa backlog 148to request the maximum allowable value, was introduced in 149.Fx 2.2 . 150